In recent years, water-absorbing resin, a typical hydrophilic polymer, is widely used for various sanitary goods such as paper diaper, sanitary napkin, adult incontinence product, water retaining agent for soil etc., and its production and consumption are both increasing. Particularly, a sanitary product such as paper diaper, sanitary napkin, or adult incontinence product is often made thin by constituting it of a large amount of water-absorbing resin and a small amount of pulp fiber. In view of this, there has been a demand for a water-absorbing resin with a large absorption against pressure, and a demand for cost reduction because a single sanitary product consumes a large amount of water-absorbing resin. For this reason, there has been a need for reduction in energy consumption and waste amount in the manufacturing line of water-absorbing resin, and also a need for finding a rational production method.
Desired properties of the water-absorbing resin include high water-absorbency, salt tolerance, low water soluble content, high absorption speed, and high gel strength. To improve these properties, some prior arts suggest a method of using a powder compound which is solid at ordinary temperature, like a specific hydrophilic high-molecular-weight compound such as starch or polyvinyl alcohol, in the process of polymerizing an unsaturated monomer in the production of the water-absorbing resin (see Documents 1 through 3). More specifically, the polymerization process has been modified many times by carrying out monomer polymerization with such a specific hydrophilic high-molecular-weight compound in order to ensure viscosity improvement of the monomer, graft polymerization, crosslinking etc.
A hydrophilic high-molecular-weight compound, which is a water-soluble high-molecular-weight compound such as polyvinyl alcohol type polymer or polyacrylamide type polymer generally has a powder form, and most of the time is dissolved in water to be an aqueous solution before added to the monomer. When such powder of water-soluble high-molecular-weight compound is supplied to ordinary-temperature water at once, it results in “fish eyes”. Therefore, the powder is generally dispersed in cold water or is gradually put into water while stirred, and then the temperature is gradually increased while stirring the solution to dissolve the powder.
However, in the foregoing method in which a hydrophilic high-molecular-weight compound solution is prepared in advance before added to the monomer solution, a process of preparing a hydrophilic high-molecular-weight compound solution has to be carried out before the step of actually preparing the water-absorbing resin, and this pre-process needs a dissolving tank with a cooling device and a heating device and a pump for transferring the resulting viscose aqueous solution, which all cause extra expense and time. Further, in actual preparation, even in the case of using “starch” which is not likely to cause “fish eyes” among the various hydrophilic high-molecular-weight compounds, it sill requires a complicated process of heating up the dispersion solution to 60° C. or more so as to produce an α-starch. Further, it is necessary to ensure that the monomer in the polymerizing aqueous solution has a high concentration so as to increase productivity and reduce cost, but it is difficult to be achieved if the hydrophilic high-molecular-weight compound solution is dilute. However, high concentration hydrophilic high-molecular-weight compound solution will make dissolving process more troublesome, and the resulting solution will be highly viscous, which is difficult to be treated. Therefore, in the recent mass production of water-absorbing resin, the method of using a hydrophilic high-molecular-weight compound in the form of hydrophilic high-molecular-weight compound solution significantly hinders improvement in productivity.
[Document 1]
Japanese Laid-Open Patent Application Tokukaisho 55-38863 (published on Mar. 18, 1980)(corresponding patent publication: Tokukosho 62-921 (published on Jan. 6, 1987)
[Document 2]
Tokukaisho 62-270607 (published on Nov. 25, 1987)(corresponding foreign patent: U.S. Pat. No. 4,826,917 (published on May 2, 1989)
[Document 3]
Tokukaisho 54-37188 (published on Mar. 19, 1979)